Scale-dependent linkages between nitrate isotopes and denitrification in surface soils: implications for isotope measurements and models
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Natural abundance nitrate (NO3 −) isotopes represent a powerful tool for assessing denitrification, yet the scale and context dependence of relationships between isotopes and denitrification have received little attention, especially in surface soils. We measured the NO3 − isotope compositions in soil extractions and lysimeter water from a semi-arid meadow and lawn during snowmelt, along with the denitrification potential, bulk O2, and a proxy for anaerobic microsites. Denitrification potential varied by three orders of magnitude and the slope of δ18O/δ15N in soil-extracted NO3 − from all samples measured 1.04 ± 0.12 (R 2 = 0.64, p < 0.0001), consistent with fractionation from denitrification. However, δ15N of extracted NO3 − was often lower than bulk soil δ15N (by up to 24 ‰), indicative of fractionation during nitrification that was partially overprinted by denitrification. Mean NO3 − isotopes in lysimeter water differed from soil extractions by up to 19 ‰ in δ18O and 12 ‰ in δ15N, indicating distinct biogeochemical processing in relatively mobile water versus soil microsites. This implies that NO3 − isotopes in streams, which are predominantly fed by mobile water, do not fully reflect terrestrial soil N cycling. Relationships between potential denitrification and δ15N of extracted NO3 − showed a strong threshold effect culminating in a null relationship at high denitrification rates. Our observations of (1) competing fractionation from nitrification and denitrification in redox-heterogeneous surface soils, (2) large NO3 − isotopic differences between relatively immobile and mobile water pools, (3) and the spatial dependence of δ18O/δ15N relationships suggest caution in using NO3 − isotopes to infer site or watershed-scale patterns in denitrification.
KeywordsIsotope mass balance model Mobile water Nitrification Redox Snowmelt
The manuscript was greatly improved by critical feedback from Jason Kaye and two anonymous reviewers. We gratefully acknowledge field and lab assistance from Simone Jackson, Jillian Turner, Dave Eiriksson, Kendalynn Morris, and contributions from Suvankar Chakraborty, Gabe Bowen, and Jim Ehleringer in implementing the denitrifier method at SIRFER. This research was supported by NSF EPSCoR grant IIA 1208732 awarded to Utah State University, as part of the State of Utah Research Infrastructure Improvement Award, and by NSF grant DBI-1337947. Any opinions, findings, and conclusions or recommendations expressed are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
Author contribution statement
S.J.H. designed the study, S.R.W. and D.R.B. contributed to sample analysis and interpretation, and S.J.H. wrote the paper with contributions from S.R.W. and D.R.B.
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